1. Field of the Invention
This invention relates to a sensor device for the selective determination of gaseous hydrocarbons.
2. Description of the Related Art
The development of easily read instruments, including portable instruments, for determining methane or other hydrocarbons is closely linked to the development of low-cost sensors of high sensitivity. From the time it was discovered that the adsorption of a gas on the surface of semiconductor oxides produces a variation in the electrical resistance of the material and that this effect is reversible, it has been attempted to utilize these properties for forming simple low-cost gas sensors. Sensors of this type are described for example in Chemical Sensor Technology, Ed. Tetsuro Seijama, Elsevier 1988. The perfection of this type of device has been rather lengthy and difficult because a sensor based on semiconductor oxide is intrinsically non-selective. In this respect it is sensitive not towards a single species but towards all those species which interact with its surface. Other problems concern stability and reproducibility, which together with selectivity represent the greatest problems to be solved before the use of such low-cost devices can become widespread.
Stability and reproducibility can be improved by improving the method of construction of the sensitive element, whereas selectivity can be improved by finding catalysts effective for a certain type of gas, by depositing suitable filters on the surface of the sensitive element and by forming an array. An array, as described for example in U.S. Pat. Nos. 4,457,161 and 4,542,640, consists of a plurality of sensors, each of which reacts differently towards the interaction with the gases contained in a determined mixture, to hence produce a different response. By processing these responses, methane or other gaseous hydrocarbons can be distinguished from other species (such as ethanol) which if only a single sensitive element were used could interfere with the measurement.
U.S. Pat. No. 4,911,892 and GB patent 2,002,907 apply the idea of using a single sensitive element but provided with a "filter" consisting of a material which favours combustion of the interfering gases, hence improving the sensitivity and selectivity of the sensor towards the species to be determined. The major drawback of this construction is that high quantities of interfering gas can cause false alarms.
European patent application publication No. 444,753 describes a hydrocarbon sensor device consisting of two sensitive elements based on amorphous semiconductor oxide, one for measurement and the other as reference, which are maintained at different operating temperatures. This method achieves discrimination between methane and interfering gases without the need for complicated processing of the signals and in particular without the need to use an array, however the sensor selectivity is unsatisfactory when the interfering gas concentration is very high.
It has now been found, according to the present invention, that by combining a sensitive measuring element, protected in the manner described hereinafter, with a sensitive reference element within a sensor based on semiconductor oxide of crystalline nature, the sensor exhibits high selectivity in determining gaseous hydrocarbons, said selectivity also being exhibited if high interfering gas concentrations are present. It therefore becomes possible to form a highly sensitive sensor in which the gaseous hydrocarbon alarm threshold, in particular for methane, is practically unaltered by the presence of interfering gases, particularly ethanol, within a wide interfering gas concentration range. The sensor device of the present invention is hence highly reliable and substantially free of the false alarm phenomena which afflict sensors of the known art.
In accordance therewith the present invention provides a semiconductor oxide-based sensor device for methane and other gaseous hydrocarbons, comprising an insulating material support carrying a heating element on one of its major surfaces, and a sensitive measuring element and a sensitive reference element on its other major surface, characterised in that:
said sensitive measuring element and said sensitive reference element are formed of crystalline tin oxide, alumina and noble metal and are in the form of thick film deposited by a silk-screen process; PA1 said sensitive measuring element is covered with a layer of electrically insulating porous material in which a noble metal is dispersed; PA1 said sensitive measuring and reference elements being heated by said heating element to mutually different temperatures and being operationally connected to an electronic comparator circuit able to measure the conductance which the sensitive elements assume by interaction with the gas, compare the signals emitted by the sensitive reference element and by the sensitive measuring element, and evaluate the presence and concentration of the hydrocarbon gas on the basis of said comparison of said signals.